MODIFICATION OF GYRE CIRCULATION
BY SUB-MESOSCALE PHYSICS
Marina Lévy1([email protected]), P. Klein2, A.-M. Tréguier2, D. Iovino1, G. Madec1, S. Masson1 and K. Takahashi3
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LOCEAN/IPSL, UPMC/CNRS/IRD/MNHN, Paris, France; 2. LPO, UBO/CNRS/IFREMER, Brest, France; 3. Earth Simulator Center, Yokohama, Japan.
Draft submitted to Ocean Modelling available at : http://www.locean-ipsl.upmc.fr/~marina/PUBLI/levy_submitted.pdf
SST
INTRODUCTION
1- MODEL & CONFIGURATION
The aim of this study is to provide a better understanding of the impact of submesoscale physics on the large scale circulation and on the large scale thermohaline equilibrium.

Rotated double-gyre configuration, (3000 × 2000 × 4) km domain with uniform
horizontal resolution on the β-plane and 30 vertical layers.
Analytical, zonal forcings (wind stress, heat and salt flux) which vary sinusoidally
between winter and summer extrema.
 4 simulations with increasing horizontal resolution:
R1 = 1°
R9 = 1/9°
R27= 1/27° R54 = 1/54°
 Simulations carried out for 100 years in order to reach equilibrium of intermediate
waters. Model means are defined as time averages over the last 10 years.

This is addressed by comparing mean characteristics of basin-scale, seasonallyvarying, subtropical and subpolar gyres in a suite of numerical experiments
varying in horizontal resolution (up to 2 km) and, accordingly, in subgrid scale
mixing.
2- EMERGENCE OF SUBMESOSCALE WITH RESOLUTION
NEMO: z-coordinate free-surface primitive equation model.

Idealized model configuration, representing
some aspects of the western North Atlantic (or
North Pacific).
At coarse resolution, the 10y-mean
barotropic circulation consists of a distinct
double-gyre structure in agreement with the
Sverdrup linear theory.. The western
boundary current (WBC) deflects at the zero
wind stress curl (∼36°N).
4- MODIFICATION OF BAROTROPIC CIRCULATION
At higher resolutions, the barotropic
circulation is intensified and strongly
deviates from the typical two-gyre structure.
The WBC deflects further south
(∼30°N
in R54). A second eastward jet emerges in
the mean circulation (located where the
wind stress reverses). Cyclonic recirculation
gyres appear in the the subtropical region.
A weak anticyclonic gyre appears in the
eastern subpolar region. These
modifications arise from the presence of the
alternative zonal jets.
Snapshots of relative vorticity at the surface show the explosion of the number of
eddies and the emergence of energetic sub-mesoscale turbulence with increasing
resolution.
3- EMERGENCE OF ZONAL JETS
5- MODIFICATION OF MIXED-LAYER DEPTH
Two important discrepancies in the MLD appear when
mesoscales and submesoscales are explicitely taken into
account:
-Decrease of the intensity of deep-convection in the
north
1y-mean
-Shallowing of the MLD at mid-latitudes (by 20-30m on
average between R9 and R54)
A significant impact of the sub-mesoscale concerns the intensification of zonal
jets with directions alternating with latitudes and mean speeds of several cm/s.
The jets are absent in R1 but occupy the whole domain in R9 and are
particularly intense close to the western boundary. They are further enhanced in
R54, particularly in the subtropical gyre.
6 - MODIFICATION OF VERTICAL DENSITY
7 - MODIFICATION OF THE MERIDIONAL OVERTURNING CIRCULATION
STRUCTURE & STRATIFICATION
In all simulations, the major feature of the MOC is a primary cell which comprises a northward
flow above 1000m, a sinking flow at around 45°N and a southward return flow at depth.
In R54, the transport associated to the MOC in the north (centered at 41°N) is strongly reduced
compared with R9 and is associated with a smaller vertical extent. This is the response to the
decrease of deep-wintertime convection. On the other hand, the strong reinforcement of the WBC
and the associated alternating zonal jets makes the secondary circulation at mid-latitudes to be
much stronger.
The vertical density structure is characterized by a bowl
shape of isopycnals in the subtropical gyre and
outcropping in the north. Resolution modifies both the
isopycnal depth and the isopycnal slope. The strenghtening
of the WBC from R1 to R54 is associated with the
steepening of the isopycnal slopes. In addition, deeper
isopycnals in the subtropical gyre and shallower
isopycnals in the subpolar gyre correspond to a southward
shift of the bowl shape.
In the internal thermocline, stratification increases by more
than a factor of two between R9 and R54. The opposite
result is found between R1 and R9.
In the mode waters, stratification increases progressively
from R1 to R9 to R54.
SUMMARY
We have computed long integrations of an idealized double-gyre circulation at sub-mesoscale resolving resolution (2 km) that allows us to demonstrate the
effect of sub-mesoscale dynamics on an equilibrated, fully prognostic density field.
When horizontal resolution is increased from eddy-resolving to sub-mesoscale resolving, a strongly turbulent eddy field emerges with the consequence of
significant modifications of the model mean fields. One original aspect of our simulations consists in the emergence of a regime of zonal jets. Another
original aspect is the restratification of the upper layers when submesoscales are taken into account. This much reduces the deep convection in the Northern
part of the domain. Furthermore, we find that the wind-driven subtropical gyre is deeper at high resolution, due to the rectifying effect of eddy fluxes.
Stratification does not vary monotonically as the model grid is refined: a stratification decrease within the internal thermocline is observed from coarse to
mesoscale resolutions, but taking into account the submesoscales leads to a significant stratification increase. Impact of submesoscales appear to reduce the
Meridional Overturning Circulation in the North - because of the restratification of the upper layers - and to locally intensify it at the latitudes of the
western boundary current - because of the emergence of the zonal jets.
ACKNOWLEDGEMENTS: This work is supported by ANR (INLOES project) and INSU-LEFE (TWISTED project), and it is part of the MOU between the Earth Simulator Center and
CNRS. The model configuration was preliminary developed by W. Hazeleger and S. Drijfhout. M.A. Foujols is thanked for developing the code on the Earth Simulator. Many thanks to R.
Benshila, C. Talandier, S. Denvil, J. Ghattas, C. Ethé. M. Kolasinski, E. Maisonnave, C. Deltel, P. Brochard, A. Caubel, P. Brockmann, F. Pinsard for running the simulations.